1. Field of the Invention
The present invention relates to a shock absorber, and more particularly to a shock absorber having a mechanical structure for adjusting damping forces to be generated by the shock absorber.
2. Prior Art
One known shock absorber with adjustable damping force comprises a cylinder, a rod slidably inserted in the cylinder, and a piston mounted on an inner end of the rod and slidably fitted in the cylinder. The piston divides the interior space of the cylinder into first and second fluid chambers. The first and second fluid chambers are connected to each other by a fluid passage whose cross-sectional area can be varied by an actuator operable by an electric signal, thus adjusting damping forces to be generated by the shock absorber.
When damping forces are changed while the shock absorber is being expanded or contracted at a high speed, a change in the produced damping forces is large, and hence the shock absorber produces impact forces which tend to impair riding comfort.
Shock absorbers are generally designed such that it produces higher damping forces when they are expanded than when they are contracted. Therefore, when damping forces are changed in an expansion stroke, large impact forces are produced which makes riding comfort poor. It is preferable that damping forces be changed when the speed at which the shock absorber is expanded or contracted is zero, i.e., when the damping forces are zero. It is more preferable that damping forces be changed the instant the expansion stroke changes to the contraction stroke and when damping forces are zero and the speed of expansion or contraction is zero, or immediately after the expansion stroke changes to the contraction stroke, i.e., the damping forces in the contraction stroke are almost zero.
Heretofore, the actuator is controlled by an electronic control unit. The electronic control unit is operated according to various control programs based on a signal from a positional sensor which detects the expansion or contraction of the shock absorber, so that damping forces can be changed at an accurate time by the electric actuator. The entire system composed of the shock absorber, the electronic control unit, and the positional sensor is however complex in structure.